1. Field of the Invention
The invention relates to optical fiber preforms. More particularly, the invention relates to Rod-In-Tube (RIT) optical fiber preform assemblies and methods for reducing movement during optical fiber manufacture.
2. Description of the Related Art
Optical fibers typically are made by heating and drawing a portion of an optical preform usually comprising a solid glass rod with a refractive glass core surrounded by a protective glass cladding. Optical fibers drawn from the preform typically are protected further by one or more coatings applied to the cladding region.
Conventionally, several processes exist for fabricating optical preforms, including modified chemical vapor deposition (MCVD), vapor axial deposition (VAD) and outside vapor deposition (OVD). These processes form a solid rod typically referred to as the preform, the preform rod, the core rod or the preform core rod. Once the preform core rod is formed, optical fiber is drawn directly therefrom or, alternatively, one or more overclad layers are formed thereon prior to drawing optical fiber therefrom. The overclad layers are formed on the preform core rod, e.g., by collapsing a silica-based overclad tube or sleeve around the preform core rod. Such process typically is referred to as the Rod-In-Tube (RIT) process. See, e.g., U.S. Pat. No. 4,820,322, which is co-owned with this application, and hereby is incorporated by reference herein.
Often, the overclad tube is collapsed onto the preform core rod while the overclad tube and the preform core rod are mounted in a vertical lathe. Alternatively, according to the Overclad During Draw (ODD) technique, the overclad tube is collapsed onto the preform core rod in the draw tower furnace, which also is used to draw optical fiber from the resulting optical fiber preform. In the ODD technique, the preform core rod is inserted into an overclad tube and then the combined preform core rod and overclad tube is moved coaxially through the draw tower furnace, which causes the collapse of the overclad tube onto the preform core rod prior to the resulting preform portion having optical fiber drawn therefrom. The ODD technique also allows for more than one overclad tube to be collapsed onto the preform core rod in the draw tower furnace. See, e.g., U.S. Pat. No. 6,460,378, which is co-owned with this application, and hereby is incorporated by reference herein.
During fiber draw, it is important that relative movement of the preform core rod and the overclad tube be minimized, since such movement often varies the geometry of the core and cladding regions, which reduces the quality of the drawn fiber. Conventionally, a silica disc is placed between the end of the preform core rod and a restricted tube-shaped handle attached to overclad tube to prevent upward movement of the preform core rod into the interior of the handle. Such movement is inherent during the fiber draw process because of the pressure gradient established across the preform core rod and the overclad tube to collapse the overclad tube onto the preform core rod.
However, as the temperature of the preform core rod increases during its movement through the draw furnace, the silica disc and the preform core rod often become soft enough to flow into the interior of the handle. Accordingly, it would be desirable to have available an optical fiber preform assembly that reduces the relative movement of the preform core and the overclad tube during the fiber draw process.